U.S. patent application number 12/046503 was filed with the patent office on 2009-05-28 for method and apparatus for detecting missing nozzle in thermal inkjet printhead.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Keon KUK, Bang-weon Lee, Seong-taek Lim.
Application Number | 20090135221 12/046503 |
Document ID | / |
Family ID | 40669334 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090135221 |
Kind Code |
A1 |
KUK; Keon ; et al. |
May 28, 2009 |
METHOD AND APPARATUS FOR DETECTING MISSING NOZZLE IN THERMAL INKJET
PRINTHEAD
Abstract
Provided is a method of detecting a missing nozzle in a thermal
inkjet printhead. The method includes: applying an input energy
high enough to eject ink to a heater corresponding to a target
nozzle, and applying an input energy not high enough to eject ink
to a hear corresponding to a nozzle adjacent to the target nozzle;
when a predetermined time passes, detecting a difference between
temperatures which are measured at points spaced by a predetermined
distance from each of the two heaters; and determining whether the
target nozzle is missing.
Inventors: |
KUK; Keon; (Yongin-si,
KR) ; Lee; Bang-weon; (Yongin-si, KR) ; Lim;
Seong-taek; (Suwon-si, KR) |
Correspondence
Address: |
STANZIONE & KIM, LLP
919 18TH STREET, N.W., SUITE 440
WASHINGTON
DC
20006
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
Suwon-si
KR
|
Family ID: |
40669334 |
Appl. No.: |
12/046503 |
Filed: |
March 12, 2008 |
Current U.S.
Class: |
347/19 |
Current CPC
Class: |
B41J 2/0451 20130101;
B41J 2/04541 20130101; B41J 2/0458 20130101; B41J 29/393
20130101 |
Class at
Publication: |
347/19 |
International
Class: |
B41J 29/393 20060101
B41J029/393 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2007 |
KR |
2007-121411 |
Claims
1. A method of detecting a missing nozzle in a thermal inkjet
printhead, the method comprising: applying an input energy high
enough to eject ink to a heater corresponding to a target nozzle,
and applying an input energy not high enough to eject ink to a hear
corresponding to a nozzle adjacent to the target nozzle; when a
predetermined time passes, detecting a difference between
temperatures which are measured at points spaced by a predetermined
distance from each of the two heaters; and determining whether the
target nozzle is missing.
2. The method of claim 1, wherein whether the target nozzle is
missing is determined by using the detected temperature
difference.
3. The method of claim 1, wherein whether target nozzle is missing
is determined by using a temperature change rate difference
calculated by using the detected temperature difference.
4. A method of detecting a missing nozzle in a thermal inkjet
printhead, the method comprising: selecting first and second
heaters adjacent to each other among heaters of the inkjet
printhead; applying a first input energy high enough to eject ink
to the first heater and applying a second input energy not high
enough to eject ink to the second heater; when a predetermined time
passes, detecting a difference between temperatures which are
measured at points spaced by a predetermined distance from each of
the first and second heaters; and determining whether the first
heater is missing.
5. The method of claim 4, wherein the second input energy is
approximately 30% of the first input energy.
6. The method of claim 4, wherein whether the first heater is
missing is determined by using the detected temperature
difference.
7. The method of claim 4, wherein whether the first heater is
missing is determined by using a temperature change rate difference
calculated by using the detected temperature difference.
8. The method of claim 4, when a predetermined time passes after
the determining of whether the first heater is missing, the method
further comprising: applying the second input energy to the first
heater and applying the first input energy to the second heater;
when a predetermined time passes, detecting a difference between
temperatures which are measured at points spaced by a predetermined
distance from each of the first and second heaters; and determining
whether the second heater is missing.
9. An apparatus for detecting a missing nozzle among nozzles of a
thermal inkjet printhead, the apparatus comprising: a plurality of
temperature measuring elements corresponding to heaters of the
inkjet printhead and spaced by predetermined distances respectively
from the heaters; a multiplexer selecting and outputting
temperatures measured by two temperature measuring elements
corresponding to the adjacent heaters from among the heaters; a
differential amplifier amplifying a difference between the
temperatures output from the multiplexer; and an
analogue-to-digital (A/D) converter connected to an output end of
the differential amplifier.
10. The apparatus of claim 9, further comprising a differential
circuit disposed between the differential amplifier and the A/D
converter and calculating a temperature change rate difference by
using the amplified temperature difference output from the
differential amplifier.
11. The apparatus of claim 9, wherein the temperature measuring
elements are metal thermometers or thermocouple thermometers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Korean Patent
Application No. 10-2007-0121411, filed on Nov. 27, 2007, in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and apparatus for
detecting a missing nozzle in an inkjet printhead, and more
particularly, to a method and apparatus for detecting a missing
nozzle in a thermal inkjet printhead.
[0004] 2. Description of the Related Art
[0005] In general, inkjet printheads are devices that eject ink
droplets onto desired positions of a recording medium to form an
image of a predetermined color. Inkjet printheads are categorized
into two types according to the ink ejection mechanism thereof. The
first one is a thermal inkjet printhead that ejects ink droplets
due to an expansion force of bubbles generated in ink by thermal
energy. The other one is a piezoelectric inkjet printhead that
ejects ink droplets due to pressure applied to ink due to
deformation of a piezoelectric body.
[0006] An ink droplet ejection mechanism of a thermal inkjet
printhead will now be explained in detail. When a pulse current is
supplied to a heater including a heating resistor, the heater
generates heat and ink near the heater is instantaneously heated up
to approximately 300.degree. C., thereby boiling the ink.
Accordingly, ink bubbles are generated by ink evaporation, and the
generated bubbles are expanded to exert pressure on the ink filled
in an ink chamber. As a result, ink around a nozzle is ejected from
the ink chamber in the form of droplets through the nozzle.
[0007] When the thermal inkjet printhead has a nozzle that leads to
poor ink ejection, streak lines are shown in a printed image,
thereby degrading print quality. Accordingly, when there is a
missing nozzle, the thermal inkjet printhead should prevent print
quality degradation by compensating for the missing nozzle with a
normal nozzle. To this end, a method of detecting a missing nozzle
by monitoring whether ink is normally ejected through nozzles of
the thermal inkjet printhead is necessary.
SUMMARY OF THE INVENTION
[0008] The present invention provides a method and apparatus for
detecting a missing nozzle in a thermal inkjet printhead.
[0009] According to an aspect of the present invention, there is
provided a method of detecting a missing nozzle in a thermal inkjet
printhead, the method comprising: applying an input energy high
enough to eject ink to a heater corresponding to a target nozzle,
and applying an input energy not high enough to eject ink to a hear
corresponding to a nozzle adjacent to the target nozzle; when a
predetermined time passes, detecting a difference between
temperatures which are measured at points spaced by a predetermined
distance from each of the two heaters; and determining whether the
target nozzle is missing.
[0010] Whether the target nozzle is missing may be determined by
using the detected temperature difference. Whether target nozzle is
missing may be determined by using a temperature change rate
difference calculated by using the detected temperature
difference.
[0011] According to another aspect of the present invention, there
is provided a method of detecting a missing nozzle in a thermal
inkjet printhead, the method comprising: selecting first and second
heaters adjacent to each other among heaters of the inkjet
printhead; applying a first input energy high enough to eject ink
to the first heater and applying a second input energy not high
enough to eject ink to the second heater; when a predetermined time
passes, detecting a difference between temperatures which are
measured at points spaced by a predetermined distance from each of
the first and second heaters; and determining whether the first
heater is missing.
[0012] The second input energy may be approximately 30% of the
first input energy.
[0013] When a predetermined time passes after the determining of
whether the first heater is missing, the method may further
comprise: applying the second input energy to the first heater and
applying the first input energy to the second heater; when a
predetermined time passes, detecting a difference between
temperatures which are measured at points spaced by a predetermined
distance from each of the first and second heaters; and determining
whether the second heater is missing.
[0014] According to another aspect of the present invention, there
is provided an apparatus for detecting a missing nozzle among
nozzles of a thermal inkjet printhead, the apparatus comprising: a
plurality of temperature measuring elements corresponding to
heaters of the inkjet printhead and spaced by predetermined
distances respectively from the heaters; a multiplexer selecting
and outputting temperatures measured by two temperature measuring
elements corresponding to the adjacent heaters from among the
heaters; a differential amplifier amplifying a difference between
the temperatures output from the multiplexer; and an
analogue-to-digital (A/D) converter connected to an output end of
the differential amplifier.
[0015] The apparatus may further comprise a differential circuit
disposed between the differential amplifier and the A/D converter
and calculating a temperature change rate difference by using the
amplified temperature difference output from the differential
amplifier.
[0016] The temperature measuring elements may be metal thermometers
or thermocouple thermometers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0018] FIG. 1 is a schematic view of an apparatus for detecting a
missing nozzle in a thermal inkjet printhead according to an
embodiment of the present invention;
[0019] FIG. 2 is a cross-sectional view taken along line II-II' of
FIG. 1;
[0020] FIG. 3 is a graph illustrating temperature and temperature
difference versus measurement distance for a normal nozzle and a
dead nozzle;
[0021] FIG. 4 is a graph illustrating temperature and temperature
difference versus for a normal nozzle and a dead nozzle when a
measurement distance is 100 .mu.m;
[0022] FIG. 5 is a graph illustrating temperature differences
between a normal nozzle and a reference nozzle and between a dead
nozzle and a reference nozzle over time using the apparatus of FIG.
1;
[0023] FIGS. 6A and 6B are schematic views for explaining a method
of detecting a missing nozzle in a thermal inkjet printhead
according to an embodiment of the present invention;
[0024] FIG. 7 is a schematic view of an apparatus for detecting a
missing nozzle in a thermal inkjet printhead according to another
embodiment of the present invention; and
[0025] FIG. 8 is a graph illustrating a temperature change rate of
a normal nozzle and a reference nozzle and a temperature change
rate of a dead nozzle and a reference nozzle over time using the
apparatus of FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The present invention will now be described more fully with
reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown. In the drawings, the same
reference numeral denote the same elements and the sizes or
thicknesses of elements may be exaggerated for clarity.
[0027] FIG. 1 is a schematic view of an apparatus for detecting a
missing nozzle in an inkjet printhead according to an embodiment of
the present invention. FIG. 2 is a cross-sectional view taken along
line II-II' of FIG. 1.
[0028] Referring to FIGS. 1 and 2, a chamber layer 120 and a nozzle
layer 130 are sequentially stacked on a substrate 110. A plurality
of ink chambers 122 in which ink to be ejected is filled are formed
in the chamber layer 120. A plurality of nozzles 132 through which
ink is ejected are formed in the nozzle layer 130. Ink feed holes
112 through which ink is supplied to the ink chambers 122 are
formed in the substrate 110. A plurality of heaters 124 for
generating bubbles by heating the ink filled in the ink chambers
112 are formed on bottom surfaces of the ink chambers 122. Although
not shown, a plurality of electrodes for supplying electric current
to the heaters 124 are formed on the heaters 124.
[0029] A plurality of temperature measuring elements 150 are formed
on the substrate 110 to be spaced by predetermined distances from
the heaters 124. The temperature measuring elements 150 may be
formed on the same plane as the heaters 124. The temperature
measuring elements 150 correspond to the heaters 124 and measure
temperatures at points spaced by predetermined distances
respectively from the heaters 124. The temperature measuring
elements 150 may be thermocouple thermometers or metal thermometers
using a resistance change. However, the present invention is not
limited thereto. In FIG. 1, X denotes a distance between an
arbitrary reference point in an ink chamber 122 and a temperature
measuring element 150.
[0030] Temperatures measured by the temperature measuring elements
150 are input to a multiplexer 160. The multiplexer 160 selects
temperatures of adjacent heaters 124 measured by two temperature
measuring elements 150 corresponding to the adjacent heaters 124
from among the heaters 124 and outputs the selected temperatures to
a differential amplifier 170. The differential amplifier 170
amplifies a difference between the temperatures measured by the two
temperature measuring elements 150 corresponding to the adjacent
heaters 124 output from the multiplexer 160 and outputs the
amplified temperature difference to an analogue-to-digital (A/D)
converter 180. In this process, since noises of the temperature
measuring elements 150 are removed by the differential amplifier
170, an accurate temperature difference can be detected. The
amplified temperature difference output to an analogue-to-digital
(A/D) converter 180 is converted into a digital signal.
[0031] A method of detecting a missing nozzle performed by the
apparatus constructed as described above according to an embodiment
of the present invention will now be explained. First, a normal
input energy high enough to eject ink is applied to a heater 124
corresponding to a target nozzle 132a whose operation is to be
measured, and an energy lower than the normal input energy, that
is, an energy not high enough to eject ink, is applied to a heater
124 corresponding to a reference nozzle 132b adjacent to the target
nozzle 132a. For example, the energy applied to the heater 124
corresponding to the reference nozzle 132b may be approximately 30%
of the normal input energy. Next, temperatures measured by
temperature measuring elements 150 corresponding to the heaters 124
are output to the multiplexer 160, and a difference between the
temperatures measured by the temperature measuring elements 150 is
detected by the differential amplifier 170 and the A/D converter
180. The difference between the temperatures of the target nozzle
132a and the reference nozzle 132b may depend on whether the target
nozzle 132a is a normal nozzle or a dead nozzle. That is, a
temperature of a normal nozzle is lower than a temperature of a
dead nozzle because of cooling effect of droplets ejected through
the normal nozzle. Accordingly, a temperature difference between a
normal nozzle and the reference nozzle 132b is smaller than a
temperature difference between a dead nozzle and the reference
nozzle 132b. Accordingly, once the temperature difference between
the target nozzle 132a and the reference nozzle 132b is measured,
whether the target nozzle 132a is a normal nozzle or a dead nozzle
can be detected. When the aforementioned process is repeated on
other remaining nozzles 132, all the nozzles 132 of the inkjet
printhead can be checked.
[0032] Temperature versus measurement distance and temperature
versus time for a normal nozzle and a dead nozzle will now be
explained with reference to FIGS. 3 and 4.
[0033] FIG. 3 is a graph illustrating temperature and temperature
difference versus measurement distance X for a normal nozzle and a
dead nozzle. Results of FIG. 3 were calculated by using heat
transfer analysis considering ink flow. A temperature difference
marked by .tangle-solidup. was obtained by subtracting a
temperature of the normal nozzle from a temperature of the dead
nozzle. The same input energy of 1.2 .mu.J was applied to heaters
124. An in ejection frequency was 6 kHz. A measurement was
conducted 0.5 seconds after ink ejection. A measurement distance X
was a distance between an arbitrary reference point in an ink
chamber 122 and a temperature measuring element 150. Referring to
FIG. 3, as the measurement distance X increases, the temperatures
of both the normal nozzle and the dead nozzle drastically decrease.
When the measurement distance X exceeds approximately 100 .mu.m, a
maximum temperature difference between the normal nozzle and the
dead nozzle is 0.16.degree. C.
[0034] FIG. 4 is a graph illustrating temperature and temperature
difference versus time for a normal nozzle and a dead nozzle when a
measurement distance X is 100 .mu.m. In FIG. 4, a temperature
difference marked by .tangle-solidup. was obtained by subtracting a
temperature of the normal nozzle from a temperature of the dead
nozzle. The same input energy of 1.2 .mu.J was applied to heaters
124. An ink ejection frequency was 6 kHz. Referring to FIG. 4, when
2 seconds pass after ink ejection, the temperatures of both the
normal nozzle and the dead nozzle rise to maximum levels, and since
then, are not changed. A temperature difference between the normal
nozzle and the dead nozzle is approximately 0.25.degree. C.
[0035] FIG. 5 is a graph illustrating temperature differences
between a normal nozzle and a reference nozzle 132b and between a
dead nozzle and the reference nozzle 132b over time when a
measurement distance X is 100 .mu.m using the apparatus of FIG. 1.
In FIG. 5, a temperature difference marked by .tangle-solidup. was
obtained by subtracting a temperature difference between the normal
nozzle and the reference nozzle 132b from a temperature difference
between the dead nozzle and the reference nozzle 132b, that is, by
subtracting a temperature of the normal nozzle from a temperature
of the dead nozzle. An input energy applied to a heater 124
corresponding to the target nozzle 132a was 1.2 .mu.J and an
ejection frequency was 6 kHz. An input energy applied to a heater
124 corresponding to the reference nozzle 132b was 30% of the input
energy applied to the target nozzle 132a.
[0036] Since the input energy applied to the reference nozzle 132b
is lower than the input energy applied to the target nozzle 132a, a
temperature of the reference nozzle 132b is lower than a
temperature of the target nozzle 132a. When 2 seconds pass after
ink ejection, the temperature of the reference nozzle 132b reaches
approximately 34.4.degree. C. Accordingly, as shown in FIG. 5, when
the target nozzle 132a is a normal nozzle, a temperature difference
T.sub.normal-T.sub.ref between the normal nozzle and the reference
nozzle 132b is approximately 1.75.degree. C., and when the target
nozzle 132a is a dead nozzle, a temperature difference
T.sub.dead-T.sub.ref between the dead nozzle and the reference
nozzle 132b is approximately 2.degree. C.
[0037] Whether the target nozzle 132a is missing can be determined
from the results of FIG. 5. In detail, when a temperature
difference T-T.sub.ref between the target nozzle 132a and the
reference nozzle 132b is a negative number, it is inferred that no
electric current is applied to the heater 124 corresponding to the
target nozzle 132a, and thus the target nozzle 132a is a missing
nozzle due to electrical short circuit. When the temperature
difference T-T.sub.ref between the target nozzle 132a and the
reference nozzle 132b is greater than 2.degree. C., it is inferred
that an input energy is applied to the heater 124 corresponding to
the target nozzle 132a, but the target nozzle 132a is a dead nozzle
not ejecting ink. When the temperature difference T-T.sub.ref
between the target nozzle 132a and the reference nozzle 132b is
less than 1.75.degree. C., it is inferred that the target nozzle
132a is a normal nozzle ejecting ink droplets each having a normal
size. When the temperature difference T-T.sub.ref between the
target nozzle 132a and the reference nozzle 132b ranges from
1.75.degree. C. to 2.degree. C., it is inferred that the target
nozzle 132a ejects ink droplets each having a size less than the
normal size.
[0038] If a temperature measuring element 150 is a metal
thermometer using a resistance change, whether the target nozzle
132a is missing may be determined by using a resistance difference
caused by a temperature difference between the target nozzle 132a
and the reference nozzle 132b as described below.
[0039] When the temperature measuring element 150 is a metal
thermometer using a resistance change, a resistance according to
temperature is expressed by
R=.alpha..times.R.sub.0.times.(T-T.sub.0)+R .sub.0 (1)
[0040] where R denotes a resistance, .alpha. denotes a temperature
coefficient of resistance, and R.sub.0 denotes a resistance at a
standard temperature, and T.sub.0 denotes the standard
temperature.
[0041] Since a distance between the target nozzle 132a and the
reference nozzle 132b which are adjacent to each other in the
thermal inkjet printhead is so small, for example, approximately 43
.mu.m, it can be assumed that the temperature coefficients of
resistance .alpha. and the resistances at the standard temperature
R.sub.0 for the adjacent target nozzle 132a and reference nozzle
132b are the same.
[0042] Accordingly, a resistance change between the target nozzle
132a and the reference nozzle 132b can be expressed by
R-R.sub.ref=.alpha..times.R.sub.0.times.(T-T.sub.ref) (2)
[0043] where R.sub.ref denotes a resistance of the reference nozzle
132b.
[0044] A resistance difference R.sub.normal-R.sub.ref between the
normal nozzle and the reference nozzle 132b and a resistance
difference R.sub.dead-R.sub.ref between the dead nozzle and the
reference nozzle 132b, which are calculated by using an aluminum
thermometer with R.sub.0 of 10 k.OMEGA. and a of 0.004403/.degree.
C. from the results of FIG. 5, are approximately 77.OMEGA. and
approximately 88.OMEGA., respectively.
[0045] Whether the target nozzle 132a is missing can be determined
from the results. In detail, when a resistance difference
R-R.sub.ref between the target nozzle 132a and the reference nozzle
132b is a negative number, it is inferred that no input energy is
applied to the target nozzle 132a and thus the target nozzle 132a
is a missing nozzle due to electrical short circuit. When the
resistance difference R-R.sub.ref between the target nozzle 132a
and the reference nozzle 132b is greater than 88.OMEGA., it is
inferred that an input energy is applied to the target nozzle 132a
but the target nozzle 132a is a dead nozzle not ejecting ink. When
the resistance difference R-R.sub.ref between the target nozzle
132a and the reference nozzle 132b is less than 77.OMEGA., it is
inferred that the target nozzle 132a is a normal nozzle ejecting
ink droplets each having a normal size. When the resistance
difference R-R.sub.ref between the target nozzle 132a and the
reference nozzle 132b ranges from 77.OMEGA. to 88.OMEGA., it is
inferred that the target nozzle 132a ejects ink droplets each
having a size less than the normal size.
[0046] A method of detecting a missing nozzle among all nozzles of
a thermal inkjet printhead will now be explained. FIGS. 6A and 6B
are schematic views for explaining a method of detecting a missing
nozzle among nozzles of a thermal inkjet printhead performed by
using the apparatus of FIG. 1 according to another embodiment of
the present invention. In FIGS. 6A and 6B, the inkjet printhead
includes 760 nozzles N1 through N760 arranged in two rows.
[0047] Referring to FIG. 6A, adjacent first and second nozzles form
one pair. For example, each of the adjacent nozzles N1 and N3, N2
and N4, N5 and N7, N6 and N8, . . . , N753 and N755, N754 and N756,
B757 and N759, and N758 and N760 form one pair. The nozzles N1, N2,
N5, N6, . . . , N753, N754, N757, N758 are first nozzles, and the
nozzles N3, N4, N7, N8, . . . , N755, N756, N758, N760 are second
nozzles. The first nozzles N1, N2, . . . , N757, N758 are set as
target nozzles whose operations are to be measured, and the second
nozzles N3, N4, . . . , N759, N760 respectively adjacent to the
first nozzles are set as reference nozzles. Accordingly, a first
input energy high enough to normally eject ink is applied to first
heaters (not shown) corresponding to the first nozzles N1, N2, . .
. , N757, N758, and a second input energy not high enough to eject
ink is applied to second heaters (not shown) corresponding to the
second nozzles N3, N4, . . . , N759, N760. The second input energy
may be approximately 30% of the first input energy.
[0048] Next, when a predetermined time, e.g., 2 seconds, passes
after ink ejection, a temperature difference or resistance
difference between the first nozzles N1, N2, . . . , N757, N758,
which are the target nozzles, and the second nozzles N3, N4, . . .
, N759, N760, which are the reference nozzles, is measured by using
the multiplexer 160 and the difference amplifier 170 of the
apparatus of FIG. 1. Whether the first nozzles N1, N2, . . . ,
N757, N758 are missing is determined by using the measured
temperature difference or resistance difference. Since a method of
determining whether a nozzle is missing by using a temperature
difference or resistance difference has already been explained in
detail, a repeated explanation will not be given.
[0049] Next, the operation of the inkjet printhead is stopped for a
predetermined period of time, e.g., 10 seconds, so that all the
nozzles N1,N2,N3,N4, . . . ,757,758,759,760 of the inkjet printhead
can reach initial temperatures.
[0050] In contrast to FIG. 6A, referring to FIG. 6B, the first
nozzles N1, N2, . . . , N757, N758 are set as reference nozzles,
and the second nozzles N3, N4, . . . , N759, N760 are set as target
nozzles. Accordingly, the first input energy high enough to
normally eject ink is applied to the second heaters corresponding
to the second nozzles N3, N4, . . . , N759, N760, and the second
input energy not high enough to eject ink is applied to the first
heaters corresponding to the first nozzles N1, N2, . . . , N757,
N758. Next, when a predetermined time, e.g., 2 seconds, passes
after ink ejection, a temperature difference or resistance
difference between the second nozzles N3, N4, . . . , N759, N760
which are the target nozzles and the first nozzles N1, N2, . . . ,
N757, N758 which are the reference nozzles is measured by using the
multiplexer 160 and the differential amplifier 170. Whether the
second nozzles N3, N4, . . . , N759, N760 are missing is determined
by using the measured temperature difference or resistance
difference. Accordingly, the method of FIGS. 6A and 6B can check
all of the nozzles N1 through N760 of the inkjet printhead and
detect whether there is a missing nozzle in the nozzles N1 through
N760.
[0051] FIG. 7 is a schematic view of an apparatus for detecting a
missing nozzle in an inkjet printhead according to another
embodiment of the present invention. The apparatus of FIG. 7 is the
same as the apparatus of FIG. 1 except that a differential circuit
190 is disposed between the differential amplifier 170 and the A/D
converter 180. Referring to FIG. 7, a temperature difference
between the target nozzle 132a and the reference nozzle 132b output
from the differential amplifier 170 is input to the differential
circuit 190. The differential circuit 190 differentiates the
temperature difference with respect to time to obtain a temperature
change rate and outputs the temperature change rate as will be
described later.
[0052] FIG. 8 is a graph illustrating a temperature change rate of
a normal nozzle and the reference nozzle 132b and a temperature
change rate of a dead nozzle and the reference nozzle 132b over
time when a measurement distance X is 100 .mu.m using the apparatus
of FIG. 7. A temperature change rate d(T.sub.normal-T.sub.ref)/dt
of the normal nozzle and the reference nozzle 132b is obtained by
differentiating a temperature difference between the normal nozzle
and the reference nozzle 132b with respect to time, and a
temperature change rate d(T.sub.dead-T.sub.ref)/dt of the dead
nozzle and the reference nozzle 132b is obtained by differentiating
a temperature difference between the dead nozzle and the reference
nozzle 132b with respect to time. In FIG. 8, a temperature change
rate difference
d(T.sub.dead-T.sub.ref)/dt-d(T.sub.normal-T.sub.ref)/dt marked by
.tangle-solidup. was obtained by subtracting the temperature change
rate d(T.sub.normal-T.sub.ref)/dt of the normal nozzle and the
reference nozzle 132b from the temperature change rate
d(T.sub.dead-T.sub.ref)/dt of the dead nozzle and the reference
nozzle 132b. Like in FIG. 5, an input energy applied to a heater
124 corresponding to the target nozzle 132a was 1.2 .mu.J and an
ejection frequency was 6 kHz. An input energy applied to a heater
124 corresponding to the reference nozzle 132b was 30% of the input
energy applied to the target nozzle 132a.
[0053] Referring to FIG. 8, when a measurement is performed at a
time indicated by an arrow, that is, when performed 70 .mu.s after
ink ejection, a minimum temperature change rate difference
d(T.sub.dead-T.sub.ref)/dt-d(T.sub.normal-T.sub.ref)/dt is
obtained. At this point of time, the temperature change rate
d(T.sub.normal-T.sub.ref)/dt of the normal nozzle and the reference
nozzle 132b is approximately 1922.degree. C./s, and the temperature
change rate d(T.sub.dead-T.sub.ref)/dt of the dead nozzle and the
reference nozzle 132b is approximately 1894.degree. C./s. Whether
the target nozzle 132a is missing can be determined by calculating
a temperature change rate d(T-T.sub.ref)/dt of the target nozzle
132a and the reference nozzle 132b from the results. In detail,
when the temperature change rate d(T-T.sub.ref)/dt between the
target nozzle 132a and the reference nozzle 132b calculated when 70
.mu.s passes after ink ejection is greater than 1922.degree. C./s,
the target nozzle 132a is a normal nozzle, and when the temperature
change rate d(T-T.sub.ref)/dt of the target nozzle 132a and the
reference nozzle 132b is less than 1894.quadrature./s, the target
nozzle 132a is a dead nozzle.
[0054] As described above, the apparatus of FIG. 7 can determine
whether the target nozzle 132a is missing by calculating a
temperature change rate of the target nozzle 132a and the reference
nozzle 132b by means of the differential circuit 190. The
calculating of the temperature change rate can be performed shortly
after ink ejection, for example, 70 .mu.s after ink ejection, a
method performed by using the apparatus of FIG. 7 according to the
present invention can reduce a measurement time considerably.
[0055] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
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